It is known in the art that ultraviolet (UV), wavelengths of light between 280 nm and 380 nm under ANSI standards and 400 nm under other AUS/NZ standards, are harmful to the human eye. More recently, it has also started to become apparent that high energy visible light, HEV, which is characterized as having wavelengths from 400 nm up to 500 nm, also poses possible threats to eye health and other living tissue. This risk occurs at the highest energies, or lowest wavelength, of the HEV spectrum. This light range has also been attributed to other beneficial biological factors such as regulation of circadian rhythms. Hence, applications are being developed to selectively reduce and or/control these high energy, UV and HEV wavelengths incident upon the eye. One approach to reducing UV and HEV wavelength incident upon the eye is to employ optical filters in or on eyewear.
Such filters can be created in a variety of ways, including through physical vapor deposition, PVD, techniques. Coatings to reduce and control the reflection from the surface of a lens, referred to as antireflection, AR, coatings are commonly employed. Examples of AR coatings can be found in U.S. Pat. No. 6,165,598 to Nelson; U.S. Pat. No. 8,425,035 to Blunckenhagen; U.S. Pat. No. 8,007,901 to Beinat; and U.S. Pat. No. 6,768,581 to Yip, which are incorporated herein in their entireties. Macleod, Angus; Thin Film Optical Filters (3rd Edition); IOP, 2001, which is incorporated herein in its entirety, provides detailed information and theory behind the design of AR coatings and optical filters.
Common considerations in the forming of AR coatings are 1) achieving high transmission throughout the visible spectrum, e.g. from 380 to 780 nm; 2) achieving a neutral or appealing reflected color; 3) controlling of off angle color; and 4) providing layers in or on the coating for improved cleanability, e.g. hydrophobicity or oleophobicity.
Generally, AR coatings employ alternating layers of at least two materials. The materials are chosen such that one material has a refractive index of less than 1.6, e.g. silicon dioxide, SiO2, and the other material has a refractive index greater than 1.6, e.g. titanium dioxide, TiO2, or, zirconium dioxide, ZrO2. More than two different materials may also be employed to achieve the desired characteristics of the coating. In addition, a conductive layer may be employed, such as indium tin oxide, as described in U.S. Pat. No. 6,852,406 to Marechal, which is incorporated herein in its entireties, or a thin metallic layer, such as 0.2 nm of Au, as described in U.S. Pat. No. 8,007,901 to Beinat, to impart anti-static properties to the coating. In these cases, the conductive layer is chosen to maximize transmission while maintaining the required anti-static property, i.e. absorption of light by the layer is minimized to be less than 5 percent through the visible, for example, less than 1 percent.
In an optical filter system, light is transmitted (T), reflected (R) or absorbed (A). If the total incident light intensity is Io then:Io=T+R+A. 
Furthermore, as described in Macleod, transmission is invariant, i.e. remains constant and does not depend on which side of a lens one is looking. Absent absorption in the substrate, e.g. optical lens material, the above relationship creates a situation in which the reflection from the front of a lens and the back of the lens must be equal. The above referenced patents do not contain any appreciable absorption in the visible portion of the spectrum since they are constructed from transparent dielectric materials. Therefore, the reflection is identical when viewed from either side of the lens, ignoring any effect of the substrate.
This poses a challenge for the design of optical filters for blocking HEV portions of the spectrum. The most common method of blocking such wavelengths is through an edge filter that has a high reflection below a specified wavelength and low reflection above this wavelength. However, achieving a high front surface reflection necessarily results in a high back surface reflection, even from a coating applied to only a front of a lens. Hence, the HEV wavelengths will be reflected into the eye from the back surface. In certain applications, this problem can become more acute. For example, the intensity of blue emission from modern HID headlamps on cars can be very distracting to drivers of oncoming cars. The blue light of the HID headlamp is scattered on surfaces leading to glare. This problem gets worse with the age of the driver. The use of an “edge” optical filter on eye glasses can help to reduce this problem by reflecting the blue light from the front of the lens. However, light from cars behind the driver will be reflected into the eyes. This is true even if the edge filter is only applied to the front of the lens, especially since most optical lens materials will not preferentially attenuate blue light. This effect can be very distracting to drivers and can even momentarily blind a driver due to passing cars.
U.S. Pat. No. 8,870,374 to Cado, which is incorporated herein in its entirety, attempts to address this problem in the context of UV light by providing front and back reflection of a lens that differ in their respective UV spectrum. This approach is based upon the UV absorption inherent in the lens material employed and upon the application of different AR coatings on the front and the back of the lens. Specifically, a low UV reflection AR is applied to the back of the lens to prevent reflection into the wearers eyes. The front AR can be independently designed and UV blocking provided by the lens material. Therefore, the ability to achieve efficient UV blocking requires the substrate or lens to provide efficient blocking characteristics. Furthermore, the designs shown in Cado have symmetric reflections from the front and back of the AR coatings from each of the coatings, ignoring any absorption in the substrate.
U.S. Pat. No. 3,679,291 to Apfel, which is incorporated herein in its entirety, describes coatings employing absorbing layers to influence the back reflection and front reflection from a surface independently. In this context “front” refers to the side of the lens containing the optical coating, i.e. one side of the coating is air and the other side of the coating is proximate the substrate or lens, and back is the surface opposing the optical coating. The design demonstrates large differences in the front and back reflections, however, with a highly undesirable transmission that is lower than 60 percent. Furthermore, the designs in Apfel are limited to broad transmission throughout the visible spectrum. U.S. Pat. No. 5,521,759 to Dobrowolski describes employing absorbing layers in a filter design with the specific purpose of side band suppression in notch transmission filters, however, also without specific concern for maintaining transmission.
Hence, there exists a need in the field for alternate filter designs and systems that independently controls the reflection of desired wavelengths of light from the front and the back surfaces of a lens, that is independent of any absorption that may be present in the lens material, that does not require application of both front side and backside coatings, and, hence, that is more economical to produce. There also exists a need in the field for alternate filter designs for other optical and aesthetic effects, such as achromaticity, without concern for increasing the back reflection into the eye.